The present invention relates to processes for aligning liquid crystals, and liquid crystal optical elements.
This invention was made with United States Government support under Agreement No. MDA972-93-2-0014 awarded by ARPA. The United States Government has certain rights in the invention.
Current liquid crystal display elements include a product that utilize a twisted nematic mode, i.e. having a structure wherein the aligning direction of nematic liquid crystal molecules is twisted by 90.degree. between a pair of upper and lower electrode substrates, a product utilizing a supertwisted nematic mode, utilizing a birefringent effect, i.e. having a structure wherein the aligning direction of nematic liquid crystal molecules is twisted by 180.degree. to 300.degree., an in-plane-switching mode wherein both electrodes controlling the liquid crystal alignment are present on one substrate and the direction of the liquid crystal orientation in the plane of the substrate changes upon application of an electric field, and a product utilizing a ferroelectric liquid crystal substance or an antiferroelectric liquid crystal substance. Common to each of these products is a liquid crystal layer disposed between a pair of substrates coated with a polymeric alignment layer. The polymeric alignment layer controls the direction of alignment of the liquid crystal medium in the absence of an electric field. Usually the direction of alignment of the liquid crystal medium is established in a mechanical buffing process wherein the polymer layer is buffed with a cloth or other fiberous material. The liquid crystal medium contacting the buffed surface typically aligns parallel to the mechanical buffing direction. Alternatively, an alignment layer comprising anisotropically absorbing molecules can be exposed to polarized light to align a liquid crystal medium as disclosed in U.S. Pat. No. 5,032,009 and 4,974,941 "Process of Aligning and Realigning Liquid Crystal Media" which are hereby incorporated by reference.
The process for aligning liquid crystal media with polarized light can be a noncontact method of alignment which has the potential to reduce dust and static charge buildup on alignment layers. Other advantages of the optical alignment process include high resolution control of alignment direction and high quality of alignment.
Requirements of optical alignment layers for liquid crystal displays include low energy threshold for alignment, transparency to visible light (no color), good dielectric properties and voltage holding ratios, long-term thermal and optical stability and in many applications a controlled uniform pre-tilt angle. Most liquid crystal devices, including displays, have a finite pre-tilt angle, controlled, for instance, by the mechanical buffing of selected polymeric alignment layers. The liquid crystal molecules in contact with such a layer aligns parallel to the buffing direction, but is not exactly parallel to the substrate. The liquid crystal molecules are slightly tilted from the substrate, for instance by about 2-15 degrees. For optimum performance in most display applications a finite and uniform pre-tilt angle of the liquid crystal is desirable.
Continuing effort has been directed to the development of processes and compositions for optical alignment of liquid crystals and liquid crystal displays. Through diligent effort and intensive experiments we have found that optical alignment of liquid crystals is especially effective when the optical alignment layers comprise polyimides containing specific structural units disclosed herein. A similar structural unit has been reported to be useful in mechanical buffing process. For instance, Japanese patent application JP 06,202,119 (C.A. 122 106745z) describes a polyimide derived from alicyclic dianhydride and 2,5-diaminobenzonitrile as a mechanically buffed alignment layer for liquid crystal displays that exhibits good electrical properties. The inventors have found that the structural units described herein exhibit unexpectedly improved alignment quality and uniformity in an optical alignment process with polarized light.